The present invention is directed to dual layer photoconductors which comprise a charge transport layer and a charge generation layer formed on a substrate. More particularly, the invention is directed to such dual layer photoconductors wherein the charge transport layer comprises binder including styrene-acrylic resin which provides the photoconductor with improved resistance to non-uniform wear, including scratching and gouging.
In electrophotography, a latent image is created on the surface of an imaging member such as a photoconducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between those areas on the surface which are exposed to light and those areas on the surface which are not exposed to light. The latent electrostatic image is developed into a visible image by electrostatic toners. The toners are selectively attracted to either the exposed or unexposed portions of the photoconductor surface, depending on the relative electrostatic charges on the photoconductor surface, the development electrode and the toner.
Although organic electrophotographic photoconductors may be of single layer construction, many organic photoconductors have a dual layer construction. Dual layer photoconductors typically comprise a substrate such as a metal ground plane member on which a charge generation layer and a charge transport layer are coated. When the charge transport layer is formed on the charge generation layer, the photoconductor exhibits a negative charge on its surface. Conversely, when the charge generation layer is formed on the charge transport layer, the photoconductor exhibits a positive charge on the surface. Conventionally, the charge generation layer comprises a polymeric binder containing a charge generating compound or molecule while the charge transport layer comprises a polymeric binder containing a charge transport compound or molecule. The charge generating compounds within the charge generation layer are sensitive to image-forming radiation and photogenerate free electron-hole pairs within the charge generation layer as a result of such radiation. The charge transport layer is usually non-absorbent of the image-forming radiation and the charge transport compounds serve to transport holes to the surface of the photoconductor. Photoconductors of this type are disclosed in the Adley et al U.S. Pat. No. 5,130,215 and the Balthis et al U.S. Pat. No. 5,545,499.
One problem associated with some organic photoconductors is that their wear performance is generally inferior to that of inorganic photoconductors, such as amorphous silicon. Photoreceptor wear in the print area is either roughly uniform or non-uniform in nature. This latter wear mechanism often appears as gouges or scratches on the photoreceptor surface, which may manifest themselves as defects in the printed product. Even thin scratches can result in a general print lightning when present in a sufficient density, or they can result in thicker printed areas when printing in duplex mode. Photoreceptor surface scratches may appear due to several factors which include: (1) interaction of an abrasive toner with the cleaning blade and the organic photoconductor surface; and/or (2) interaction of paper with the organic photoconductor surface. The abrasive components of the toner are key contributors to the level of scratching: common toner additives such as silicon carbide are extremely hard, and thus more prone to scratch the photoconductor surface.
A known approach to decreasing the wear and scratching of an organic photoconductor is to provide an additional hardened overcoating which is designed to make the organic photoconductor harder and thus more wear resistant. This additional layer, however, adds additional expense and an additional manufacturing step. Consequently, a need exists for providing organic photoconductors that exhibit improved wear characteristics without adversely affecting the electrical properties of the photoconductor or significantly increasing cost or manufacturing complexity of the photoconductors.
Accordingly, it is an object of the present invention to provide improved photoconductors and improved charge transport layers for use in photoconductors. More particularly, it is an object of the present invention to provide charge transport layers and dual layer photoconductors which exhibit improved resistance to non-uniform wear, for example scratching and gouging, which may detract from printed images, while maintaining good electrical performance and acceptable durability.
These and additional objects and advantages are provided by the charge transport layers and the dual layer photoconductors according to the present invention. The charge transport layers according to the invention comprise charge transport compound and binder including styrene-acrylic resin. The photoconductors according to the present invention comprise a substrate, a charge generation layer and a charge transport layer, wherein the charge transport layer comprises charge transport compound and binder including styrene-acrylic resin. In another embodiment, photoconductors according to the present invention comprise a substrate; a charge generation layer formed on the substrate and comprising charge generation compound and charge generation layer binder, wherein the charge generation compound comprises titanylphthalocyanine; and c) a charge transport layer formed on the charge generation layer and comprising charge transport compound and binder including polycarbonate and styrene-acrylic resin, wherein the charge transport compound comprises a hydrazone compound, and wherein the styrene-acrylic resin is present in an amount sufficient to improve non-uniform wear resistance of the charge transport layer.
Styrene-acrylic resins, especially those containing a high percentage of styrene, are well known as soft polymers. Surprisingly, the addition of such a soft polymer resin to the charge transport layer of an organic photoconductor decreases or eliminates non-uniform wear such as scratching or gouging on the photoconductor surface. Furthermore, the dual layer photoconductors according to the present invention are advantageous in that they exhibit good electrical performance, including good sensitivity and/or good residual voltage.
These and additional objects and advantages will be further apparent in view of the following detailed description.
The charge transport layers according to the present invention comprise charge transport compound and binder including styrene-acrylic resin. The dual layer photoconductors according to the present invention comprise a substrate, a charge generation layer and a charge transport layer. The charge transport layer comprises charge transport compound and binder including styrene-acrylic resin. The charge generation layer typically comprises charge generating compound and binder.
The photoconductor substrate may be flexible, for example in the form of a flexible web or a belt, or inflexible, for example in the form of a drum. Typically, the photoconductor substrate is uniformly coated with a thin layer of a metal, which functions as an electrical ground plate. In one embodiment, this metal layer is aluminum. In a further embodiment, the aluminum is anodized to convert the aluminum surface into a thicker aluminum oxide surface. Alternatively, the ground plane member may comprise a metallic plate, such as aluminum or nickel, a metallic drum or foil, or a plastic film on which aluminum, tin oxide or indium oxide or the like is vacuum evaporated.
Typically the charge generation layer may be formed on the photoconductor substrate, followed by formation of the charge transport layer, whereby the photoconductor surface exhibits a negative charge and the non-uniform wear resistance benefits of the charge transport layer are maximized.
The charge transport layer included in the dual layer photoconductors according to the present invention comprises charge transport compound and binder including styrene-acrylic resin. The charge transport layer may include one or more of any of the charge transport compounds generally known in the art for use in charge transport layers. Charge transport compounds suitable for use in the charge transport layer of the photoconductors of the present invention should be capable of supporting the injection of photo-generated holes and electrons from the charge generation layer and allowing the transport of these holes or electrons through the charge transport layer to selectively discharge the surface charge. Suitable charge transport compounds for use in the charge transport layer include, but are not limited to, the following:
1. Diamine and triarylamine transport molecules of the types described in U.S. Pat. Nos. 4,306,008, 4,304,829, 4,233,384, 4,115,116, 4,299,897, 4,265,990 and/or 4,081,274. Typical diamine transport molecules include N,Nxe2x80x2-diphenyl-N,Nxe2x80x2-bis(alkylphenyl)-[1,1xe2x80x2-biphenyl]-4,4xe2x80x2-diamines wherein the alkyl is, for example, methyl, ethyl, propyl, n-butyl, or the like, or halogen substituted derivatives thereof, commonly referred to as benzidine and substituted benzidine compounds, and the like. Typical triarylamines include, for example, tritolylamine, and the like.
2. Pyrazoline transport molecules as disclosed in U.S. Pat. Nos. 4,315,982, 4,278,746 and 3,837,851. Typical pyrazoline transport molecules include 1-[lepidyl-(2)]-3-(p-diethylaminophenyl)-5-p-diethylaminophenyl)pyrazoline, 1-[quinolyl-(2)]-3-(p-diethylaminophenyl)-5-(p-diethylaminophenyl)pyrazoline, 1-[pyridyl-(2)]-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl)pyrazoline, 1-[6-methoxypyridyl-(2)]-3-(p-diethylaminostyryl)-5-(p-diethylaminophenyl) pyrazoline, 1-phenyl-3-[p-diethylaminostyryl]-5-(p-dimethylaminostyryl)pyrazoline, 1-phenyl-3-[p-diethylaminostyryl]-5-(p-diethylaminostyryl)pyrazoline, and the like.
3. Substituted fluorene charge transport molecules as described in U.S. Pat. No. 4,245,021. Typical fluorene charge transport molecules include 9-(4xe2x80x2-dimethylaminobenzylidene)fluorene, 9-(4xe2x80x2-methoxybenzylidene)fluorene, 9-(2,4xe2x80x2-dimethoxybenzylidene)fluorene, 2-nitro-9-benzylidene-fluorene, 2-nitro-9-(4xe2x80x2-diethylaminobenzylidene)fluorene and the like.
4. Oxadiazole transport molecules such as 2,5-bis(4-diethylaminophenyl)-1,3,4-oxadiazole, imidazole, triazole, and others as described in German Patents Nos. 1,058,836, 1,060,260 and 1,120,875 and U.S. Pat. No. 3,895,944.
5. Hydrazone transport molecules including p-diethylaminobenzaldehyde-(diphenylhydrazone), p-diphenylaminobenzaldehyde-(diphenylhydrazone), o-ethoxy-p-diethylaminobenzaldehyde-(diphenylhydrazone), o-methyl-p-diethylaminobenzaldehyde-(diphenylhydrazone), o-methyl-p-dimethylaminobenzaldehyde(diphenylhydrazone), p-dipropylaminobenzaldehyde-(diphenylhydrazone), p-diethylaminobenzaldehyde-(benzylphenylhydrazone), p-dibutylaminobenzaldehyde-(diphenylhydrazone), p-dimethylaminobenzaldehyde-(diphenylhydrazone) and the like described, for example, in U.S. Pat. No. 4,150,987. Other hydrazone transport molecules include compounds such as 1-naphthalenecarbaldehyde 1-methyl-1-phenylhydrazone, 1-naphthalenecarbaldehyde 1,1-phenylhydrazone, 4-methoxynaphthlene-1-carbaldehyde 1-methyl-1-phenylhydrazone and other hydrazone transport molecules described, for example, in U.S. Pat. Nos. 4,385,106, 4,338,388, 4,387,147, 4,399,208 and 4,399,207. Yet other hydrazone charge transport molecules include carbazole phenyl hydrazones such a 9-methylcarbazole-3-carbaldehyde-1,1-diphenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-methyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-ethyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1-ethyl-1-benzyl-1-phenylhydrazone, 9-ethylcarbazole-3-carbaldehyde-1,1-diphenylhydrazone, and other suitable carbazole phenyl hydrazone transport molecules described, for example, in U.S. Pat. No. 4,256,821. Similar hydrazone transport molecules are described, for example, in U.S. Pat. No. 4,297,426. Hydrazone transport molecules, suitable for use in the charge transport layer, include derivatives of aminobenzaldehydes, cinnamic esters or hydroxylated benzaldehydes. Exemplary amino benzaldehyde-derived hydrazones include those set forth in the Anderson et al U.S. Pat. Nos. 4,150,987 and 4,362,798, while exemplary cinnamic ester-derived hydrazones and hydroxylated benzaldehyde-derived hydrazones are set forth in the copending Levin et al U.S. application Ser. No. 08/988,600 and U.S. Pat. No. 5,925,486, respectively, all of which patents and applications are incorporated herein by reference.
In a specific embodiment, the charge transport compound is selected from the group consisting of diamine transport compounds, pyrazoline transport compounds, substituted fluorine transport compounds, hydrazone transport compounds, and mixtures thereof. In a further embodiment, the charge transport compound included in the charge transport layer comprises a hydrazone, an aromatic amine (including aromatic diamines), a substituted aromatic amine (including substituted aromatic diamines), or a mixture thereof. In yet a further embodiment, the charge transport compound comprises a hydrazone transport compound.
The charge transport layer typically comprises charge transport compound in an amount of from about 5 to about 60 weight percent, based on the weight of the charge transport layer. In a more specific embodiment, the charge transport layer comprises charge transport compound in an amount of from about 15 to about 40 weight percent, based on the weight of the charge transport layer, with the remainder of the charge transport layer typically comprising the binder including styrene-acrylic resin, although other additional components conventionally employed in charge transport layers may be included therein. Thus, the charge transport layer may comprise from about 40 to about 95 weight percent of the binder and, in a more specific embodiment, may comprise from about 60 to about 85 weight percent of the binder.
Typically, the polymeric binder of the charge transport layer should be inactive, i.e., not exhibiting charge transporting properties. In one embodiment, the binder of the charge transport layer comprises, in addition to the styrene-acrylic resin, a resin exhibiting a hardness greater than the hardness of the styrene-acrylic resin. In another embodiment, the binder is polymeric and, in addition to styrene-acrylic resin, may comprise, but is not limited to, polycarbonate polymers and copolymers, including polyestercarbonates, vinyl polymers such as polyvinyl chloride, polyvinyl butyryl, polyvinyl acetate, other styrene polymers, and copolymers of these vinyl polymers, other acrylic acid and acrylate polymers and copolymers, polyesters, alkyd resins, polyamides, polyurethanes, epoxy resins and the like. In a specific embodiment, the binder comprises polycarbonate in combination with the styrene-acrylic resin.
The styrene-acrylic resin included in the binder of the charge transport layers of the present invention comprises a copolymer of styrene monomer and acrylic monomer. The term xe2x80x9cstyrene monomerxe2x80x9d as used herein includes all aromatic vinyl monomers, including but not limited to, styrene and substituted styrene including one or more substituents such as alkyl, alkoxy, halogen, and the like. Suitable alkyl and alkoxy substituents may include, for example, 1 to 10 carbon atoms. The term xe2x80x9cacrylic monomerxe2x80x9d as used herein includes acrylic acids, such as acrylic acid and/or methacrylic acid and alkyl acrylate esters thereof, including but not limited to ethyl acrylate, butyl acrylate, methyl acrylate, butyl methacrylate, ethyl methacrylate, methyl methacrylate and the like. Alkyl groups may include, for example, 1 to 10 carbon atoms. In a more specific embodiment, the styrene-acrylic resin is formed from styrene monomer and butyl acrylate monomer.
The styrene-acrylic resin typically comprises from about 5% to about 95% styrene monomer units, by weight, and from about 5% to about 95% acrylic monomer units, by weight. In a more specific embodiment, the styrene-acrylic resin comprises from about 50% to about 90% styrene monomer units, and from about 10% to about 50% acrylic monomer units. In a further embodiment, the styrene-acrylic resin comprises from about 60% to about 90% styrene monomer units, and from about 10% to about 40% acrylic monomer units.
In one embodiment, the styrene-acrylic resin has a high molecular weight, or a weight average molecular weight of at least about 250,000. In a further embodiment, the styrene-acrylic resin has a weight average molecular weight of at least about 1,000,000. The only upper limit to the molecular weight of the styrene-acrylic resin is the molecular weight at which the resin is no longer soluble in a solvent commonly used to form charge transport layers, for example, tetrahydrofuran. In a further embodiment, the styrene-acrylic resin is a monomodal polymer.
In another embodiment, the styrene-acrylic resin has about a 0% gel content, thereby indicating that the material is not measurably cross-linked. Consequently, solubility of the resin and subsequent coating of the charge transport layer upon the charge generation layer are thus facilitated.
In yet another embodiment, the styrene-acrylic resin has an acid content less than about 0.5%, based on the weight of the resin. In a more specific embodiment, the styrene-acrylic resin has an acid content less than about 0.2%, based on the weight of the resin. In yet a more specific embodiment, the styrene-acrylic resin has an acid content less than about 0.1%, based on the weight of the resin. The acid content of the resin may be determined by titration in methanol using KOH as a base and phenylthaline as an indicator, in accordance with well-known techniques.
The binder of the charge transport layer may include the styrene-acrylic resin in any desired amount. In one embodiment, the charge transport layer binder includes the styrene-acrylic resin an amount sufficient to improve the non-uniform wear of the layer. Improvement in non-uniform wear may be evaluated in terms of improved scratch resistance and/or improved gouge resistance of the charge transport layer. In a further embodiment, the charge transport layer comprises from about 1% to about 15% of the styrene-acrylic resin, by weight of the charge transport layer. In yet a further embodiment. the charge transport layer comprises from about 1% to about 10% of the styrene-acrylic resin, by weight of the charge transport layer.
While not wishing to be bound by theory, it is believed that the styrene-acrylic separates into spherical domains, and that regions of soft styrene-acrylic and harder polycarbonate may contribute to improved scratching resistance.
As set forth above, the charge generation layer may comprise charge generating compound and binder. Various charge generation compounds which are known in the art are suitable for use in the charge generation layer of the photoconductors according to the present invention. Organic charge generation compounds are suitable for use in the present photoconductors, examples of which include, but are not limited to, disazo compounds, for example as disclosed in the Ishikawa et al U.S. Pat. No. 4,413,045, tris and tetrakis compounds as known in the art, phthalocyanine dyes, including both metal-free forms such as X-form metal-free phthalocyanines and the metal-containing phthalocyanines such as titanium-containing phthalocyanines as disclosed in U.S. Pat. Nos. 4,664,997, 4,725,519 and 4,777,251, polymorphs and derivatives thereof, and squaric acid-derived dyes, for example hydroxy-squaraine charge generation compounds. In one embodiment, the charge generation compound is selected from the group consisting of disazo compounds, tris and tetrakis compounds, phthalocyanine dyes, polymorphs and derivatives thereof, squaric acid-derived dyes, and mixtures thereof.
In a more specific embodiment, the charge generation compound for use in the charge generation layer according to the present invention comprises metal-containing phthalocyanines, and more particularly metal-containing phthalocyanines wherein the metal is a transition metal or a group IIIA metal. In a further embodiment, the charge generation compound comprises metal-containing phthalocyanine containing a transition metal such as copper, titanium or manganese or containing aluminum as a group IIIA metal. The metal-containing phthalocyanine charge generation compound optionally may be oxy, thiol or dihalo substituted. In yet a further embodiment, the charge generation compound comprises a titanylphthalocyanine. In yet a further embodiment, charge generation compounds in the charge generation layer comprise titanylphthalocyanines, including various polymorphs thereof, for example type IV polymorphs, and derivatives thereof, for example halogen-substituted derivatives such as chlorotitanyl phthalocyanines.
The charge generating compounds are employed in the charge generation layer in conventional amounts suitable for providing the charge generation effects. In one embodiment, the charge generation layer comprises charge generation compound in an amount of from about 10% to about 90%, by weight of the charge generation layer. In another embodiment, the charge generation layer comprises charge generation compound in an amount of from about 25% to about 80% by weight of the charge generation layer.
The polymeric binder of the charge generation layer may be any polymeric binder known in the art for use in charge generation layers. Typically, the binder of the charge generation layer should be inactive, i.e, not exhibiting either charge generation or charge transporting properties. The charge generation layer binder may comprise, but is not limited to, polycarbonate polymers and copolymers, including polyestercarbonates, vinyl polymers such as polyvinyl chloride, polyvinyl butyryl, polyvinyl acetate, styrene polymers, and copolymers of these vinyl polymers, acrylic acid and acrylate polymers and copolymers, polyesters, alkyd resins, polyamides, polyurethanes, epoxy resins and the like.
In another embodiment, the charge generation layer comprises the binder in an amount of from about 10% to about 90% by weight of the charge generation layer. In a further embodiment, the charge generation layer comprises the binder in an amount of from about 20% to about 75% by weight of the charge generation layer.
In a further specific embodiment, the photoconductor of the present invention comprises a substrate; a charge generation layer formed on the substrate and comprising charge generation compound and charge generation layer binder, wherein the charge generation compound comprises titanylphthalocyanine; and a charge transport layer formed on the charge generation layer and comprising charge transport compound and binder including polycarbonate and styrene-acrylic resin; wherein the charge transport compound comprises a hydrazone compound, and wherein styrene-acrylic resin is present in an amount sufficient to improve non-uniform wear resistance.
The photoconductor imaging members described herein may be prepared according to conventional techniques. Typically, the anodized layer of the aluminum photoconductor substrate will have a thickness of from about 3 microns to about 9 microns, the charge generation layer will have a thickness of from about 0.05 to about 5 microns, and the charge transport layer will have a thickness of from about 10 to about 35 microns. In accordance with techniques known in the art, a barrier layer may be provided between the ground plane and the charge generation layer, typically having a thickness of from about 0.05 to about 2.0 microns. The charge generation layer may be formed by dispersing the charge generating compound in a polymeric binder and solvent, coating the dispersion on the respective underlying layer and drying the coating. Similarly, the charge transport layer may be formed by dispersing the charge transport compound and a polymeric binder including styrene-acrylic resin in solvent, coating the dispersion on the respective underlying layer and drying the coating.
Various embodiments of the photoconductors according to the present invention are illustrated in the following examples. In the examples and throughout the present specification, parts and percentages are by weight unless otherwise specified.